Coforge Java Developer Interview Questions and Answers

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Q1. Explain how hashmap internally works
Ans. 
HashMap internally uses an array of linked lists to store key-value pairs.
• HashMap uses hashing to determine the index of the key in the array.

• If multiple keys hash to the same index, a linked list is used to store them.

• When retrieving a value, the key is hashed to find the index and then the linked list is searched for the key.
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Q2. Difference between vector and Arraylist
Ans. 
Vector is synchronized and slower, ArrayList is unsynchronized and faster.
• Vector is synchronized, ArrayList is not.

• Vector is slower due to synchronization, ArrayList is faster.

• Vector is legacy class, ArrayList is part of Java Collections framework.

• Vector doubles its size when full, ArrayList increases by 50%.
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Aptitude questions and some questions from PostgreSQL.

Q1. Java fundamental questions?
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Q2. Collection Framework.
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Q3. 2-3 easy to medium Coding Questions.
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Q4. Questions from Object oriented programming in Java?
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Q5. Implementation of HashMap , LinkedList, HashSet, Heap?
Ans. 
HashMap, LinkedList, HashSet, and Heap are data structures commonly used in Java for storing and organizing data.
• HashMap: key-value pairs, uses hashing to store and retrieve elements efficiently (e.g. HashMap)

• LinkedList: linear data structure, elements are stored in nodes with pointers to the next node (e.g. LinkedList)

• HashSet: collection of unique elements, uses hashing to ensure uniqueness (e.g. Hash...read more
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Q1. Comparable vs Comparator
Ans. 
Comparable is an interface used for natural ordering, while Comparator is used for custom ordering in Java.
• Comparable interface is used to define the natural ordering of objects. It is implemented by the class whose objects are to be sorted.

• Comparator interface is used to define custom ordering of objects. It is implemented by a separate class that compares objects based on specific criteria.

• Example: Sorting a list of ...read more
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Q2. Difference between List and Set

Ans. 

List allows duplicate elements and maintains insertion order, while Set does not allow duplicates and does not maintain order.

• List can contain duplicate elements, Set cannot

• List maintains insertion order, Set does not guarantee order

• List is implemented by classes like ArrayList, LinkedList, Set is implemented by classes like HashSet, TreeSet

• Example: List list = new ArrayList<>(); Set set = new HashSet<>();

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